MrSeb writes: "In a twist that's very reminiscent of Microsoft's naughty nineties, Mozilla has revealed that Windows RT — aka Windows 8 on ARM — will only support one web browser: Internet Explorer. While Firefox will technically be able to run in Metro mode on Windows RT, it will be so crippled as to be unusable; in "classic," Desktop mode, third-party browsers such as Firefox won't be allowed to run at all. This restriction seems to stem from Microsoft locking down some vital APIs in Windows RT, so that it's impossible to build a browser that competes with Internet Explorer. At first blush this sounds like a classic ploy to stymy the opposition and regain market share — and for all I know, maybe it is — but there are also a few logical reasons for Microsoft's decision. From the get-go, Microsoft has been leery of developers porting x86 code to ARM, in case these ported apps don't have the efficiency and stability that a low-power (and battery-powered) ARM tablet requires. It's for this reason that Microsoft didn't offer some kind of OS- or hardware-level x86>ARM translation. For users, however, the implications are extensive: Windows RT won't have browser add-ons, or web apps, or the ability to use WebGL sites. In the long term, if Windows on ARM is a big success, we could even return to the mid-'90s digital dark age, where more than 90% of web surfers used Internet Explorer — a fate worse than death itself."Link to Original Source

It seems in Windows on ARM, third-party apps will only have access to the WinRT (Metro) API, while Microsoft's own software (such as IE10) will have access to Win32. Without Win32, Firefox (or Chrome, or..) won't be able to do things such as JIT compilation of JavaScript, and other low-level stuffs.